The lung tolerance in mice after single and fractionated irradiations with p(45)+Be and p(65)+Be neutrons produced at the isochronous cyclotron "CYCLONE" of Louvain-la-Neuve (Belgium) was studied. Cobalt-60 gamma rays were used for control irradiations. The end point was the dose which was lethal to 50% of the mice by 180 days (LD50/180). On a log-log plot, the slope (+/- SE) of the relationship between total isoeffect dose and fraction number decreases from 0.34 +/- 0.01 for gamma rays to 0.19 +/- 0.01 for p(65)+Be and 0.12 +/- 0.01 for p(45)+Be neutrons. The data have been analyzed using the linear-quadratic (LQ) model. The alpha/beta ratio (+95% confidence interval) increases from 5.3 (4.3-6.4) for gamma rays to 20.7 (16.7-24.9) for p(65)+Be and 37.9 (25.8-65.8) for p(45)+Be. The RBEs of neutrons relative to gamma rays were estimated from the LQ parameters, to 1.15 and 1.19 for a dose of 14 Gy gamma rays and 2.02 and 2.47 for a dose of 2 Gy gamma rays for p(65)+Be and p(45)+Be neutrons, respectively. The neutron RBE of the p(45)+Be relative to the p(65)+Be calculated from the ratio of their respective RBEs relative to gamma rays reaches 1.03 and 1.23 for doses of 14 and 2 Gy gamma-ray equivalent, respectively. These data are compared with other published data on lung tolerance after irradiation with lower-energy neutrons and with data obtained previously in our laboratory on mouse jejunum and Vicia faba.